This invention relates to a gas component detector made of metal oxide. It is used, for instance, in controlling the components of gas exhausted from an internal combustion engine to within predetermined values in order to render the gas harmless. More particularly, this invention relates to a gas component detector in which the assembly structure of a resistance variation type gas detector and a cylindrical ceramic heater is improved. Also, the labor, time and cost required for assembling and manufacturing the device are reduced, and deterioration of the dynamic response of assembled parts is prevented.
Solid electrolytic gas sensors using sheath heaters are disclosed in Japanese Patent Application Laid-Open Nos. 134495/1979 and 21393/1979 and Japanese Utility Model Application Laid-Open No. 39087/1979. A resistance variation type gas detector incorporating a cylindrical ceramic heater is disclosed in Japanese Utility Model Application No. 105933/1980 and have been proposed as gas component detectors. In such gas component detectors, a heater is employed to heat the sensor so that the characteristics of the sensor will not be adversely affected by temperature variations, especially by low temperature. This will now be described with reference to FIG. 1 which is a longitudinal sectional view of the aforementioned resistance variation type gas detector disclosed, by way of example, in Japanese Utility Model Application No. 105933/1980.
In the gas detector of FIG. 1, a gas component detecting element 1 made of sintered metal oxide, a pair of electrodes 2 for transmitting an electrical resistance indicated by the gas component detecting element 1, and a ceramic tube 3 which insulates the pair of electrodes and holds the detecting element are formed into one unit with heat-resisting cement 6. The unit is inserted into a sintered cylindrical ceramic heater 4 which is separately manufactured. The space between the ceramic tube 3 and the ceramic heater 4 is filled with heat-resisting cement 6, so that the former is bonded to the latter. The resultant assembly is then dried. The assembly includes a heat-generating wire 5 as shown in FIG. 1. The assembly is protected from damage by a metal portion 7 and a protector sheath 8 having gas discharging holes 9, in use.
In FIG. 1, reference numeral 10 designates a holding metal sleeve. In the detector thus constructed, the cylindrical ceramic heater is manufactured in addition to the ceramic tube. While manufacture of the cylindrical ceramic heater is not costly per se, the heater is strained during the sintering process. Therefore, when the ceramic tube is inserted into the cylindrical ceramic heater, the gap between the tube and the heater is not uniform. Accordingly, considerable time and labor are needed to fill the gap with heat-resisting cement, and it is difficult to completely fill the gap therewith therefore many voids are remained at the refractory cement. Thus, the resulting heater assembly has a high thermal resistance between the sensor element and the ceramic heater, then the effective heating of the sensor element cannot be realized. Furtheremore, since gas exhausted from an internal combustion engine is left in the void, the response of the detecting element deteriorates.